These days, most of the InGaAlN (a group III nitride semiconductor) devices used for ultraviolet, violet, blue and green optical sources are formed on a substrate of sapphire or silicon carbide (SiC) by conducting thereon an MOCVD process (metal-organic chemical vapor deposition process) or MBE process (molecular beam epitaxy process). In such a case, however, a large amount of crystal defects are included in the group III nitride crystal in view of large difference of thermal expansion coefficient and lattice constant between the substrate and the group III nitride semiconductor layers. In the case of a light-emitting device, for example, problems such as short lifetime, large operational power consumption, and the like, are caused as a direct consequence of such crystal defects existing with high density.